A pseudopotential model for the description of binding of an excess electron
to polar clusters or molecules is presented. In addition to Coulomb, short range
repulsion, and polarization interactions between the excess electron and the neutral core,
the model also accounts for dispersion within a second order perturbation treatment.
The pseudopotential, which should enable future dynamical calculations coupling
the excess electron with nuclear motions, is successfully tested against accurate
ab initio results for a whole set of geometries of hydrogen fluoride dimer
anion. Predictions are made for an electron bound to a collinear hydrogen fluoride trimer
for different values of the intermonomer separations. For the optimal and shorter values
of this separation two bound states of the excess electron in (HF)3-
are predicted to exist.